Adult stem cells from bone marrow have the potential to provide a vast resource for regenerative medicine which would allow the replacement of injured or defective cells and tissues. While it is known that mesenchymal stem cell (MSC) populations from bone marrow have the potential to differentiate into many different cell types including bone, cartilage, fat and possibly others, there is little understanding of the molecular basis that characterizes the different progenitor cells. Also lacking is a clear understanding of the factors that regulate their growth and differentiation. The purpose of this grant proposal is to identify and characterize natural ligands from within the human genome that target cell surface receptors on mesenchymal stem cells. Phage display libraries will be constructed using novel cDNA display methods and selected on MSCs to identify natural peptide ligands that bind and internalize into MSCs. Novel selection methods will be used to select internalizing ligands with greater sensitivity than currently available techniques. Selected ligands will be characterized for specificity and function. Near term, these ligands will serve as targeting ligands for the introduction of genes and drugs into mesenchymal stem cells that would augment their use in tissue engineering and treatment of disease. Moreover, it is likely that certain ligands will be useful as biological drugs that can be used commercially to either maintain stem cell populations in an undifferentiated state, or to stimulate their differentiation to specific cell types. In the long term, an understanding of these natural ligands and their receptors on bone marrow stem cells will allow rational design of drug treatments that augment the bodies own mechanism for tissue repair due to injury (i.e. wound healing, bone and cartilage growth), or due to cell loss from diseases like diabetes, Parkinson's disease, and muscular dystrophy.